Such an opaque, low density, porous sheet or film is useful as a printing substrate, such as synthetic paper; as a substitute for leather; as a highly fibrillated sheet which can easily be shredded into fine fibrils to be used as substitutes for paper-making pulps, or as a filter material, such as battery separators.
Many polymeric materials or especially blends thereof are known to undergo fibrillation and/or pore formation upon stretching or drawing. A number of such blends are described in U.S. Pat. Nos. 3,697,367 to Schwarz and 3,511,742 to Rasmussen. Such pore formation may result from different causes, such as separation of phases of incompatible polymer blends, or separation of inorganic polymer fillers like clay or titanium dioxide from the polymer matrix due to stress concentration. Most common in such systems is that the maximum pore formation effect occurs at a draw temperature which is relatively low for the particular polymer system. When the same polymer or blend thereof is stretched at higher temperatures, the pore formation diminishes and a denser film results.
At temperatures where pore formation occurs accompanied by a decrease in density, the draw tension also increases. Draw tension or yield strain also increases with increasing draw rate or operating speed, and reaches the breaking strength of the base film at speeds which are slow and uneconomical for conventional systems used for stretching or drawing of films. Operating a conventional stretching system, such as longitudinal stretching by Godet rolls and lateral stretching by tenter frames, under tensions which approach the breaking strength of the base film often causes breaks and frequent interruptions of the process. Extrusion speeds are uneconomically slow: for instance, an acceptable draw rate of 200 cm/min in a single longitudinal draw step over Godet rolls for a 90 wt% isotatic polypropylene - 10 wt% polystyrene (See Example 1), would limit the extrusion rate (for a 3 foot linear die at a draw ratio of 2.0 and a film thickness of 100 micron) to 23.2 lb/hr.
The process heretofore advanced for making porous films are plagued, inter alia, by low production rates, low yields, nonuniform quality, etc.